Broncos outside linebacker Von Miller finished second in defensive MVP voting in 2012 after a season in which he had 18.5 sacks. / Ron Chenoy, USA TODAY Sports

by Lindsay H. Jones, USA TODAY Sports

by Lindsay H. Jones, USA TODAY Sports

The Denver Broncos could lose their best defensive player for the start of the 2013 NFL season.

All-pro outside linebacker Von Miller is appealing a four-game suspension for a violation of a league policy. The NFL Players Association confirmed it is assisting Miller in the appeal, which union spokesman George Atallah said is not for performance enhancing drugs.

The Denver Post, citing anonymous sources, reported that Miller tested positive for marijuana and amphetamines in 2011, his rookie season. Under the NFL's substance abuse policy, players who test positive for recreational or illegal prescription drugs are not suspended after the first positive test but are punished for positive tests once in the league's drug program.

Miller, posting via his verified Twitter account, claimed innocence, and wrote that he was disappointed his team will "open camp like this."

"I know I did nothing wrong," Miller wrote. "I'm sure this will be resolved fairly."

The Broncos, citing confidentiality laws, declined comment, but team spokesman Patrick Smyth told USA TODAY Sports the team was aware of the ESPN report.

Messages left for Miller's agent at Athletes First have not been returned.

Atallah said via Twitter that the union is "working closely" with Miller to appeal the suspension. "We are all very disappointed that confidentiality was breached," Atallah wrote.

The NFL and NFLPA do not announce suspensions until after the appeals process is concluded, and those who violate confidentiality can be fined at least $10,000.

Miller, the No. 2 overall pick of the 2011 draft, finished second in defensive MVP voting in 2012 after a season in which he had 18½ sacks. He has 30 sacks in two seasons.

With the loss of defensive end Elvis Dumervil in free agency following a fiasco with a fax machine in March, the Broncos were looking for Miller to increase his production and take a larger leadership role. His potential suspension would be an even bigger blow.

If Miller's appeal is not successful, the Broncos will play the Baltimore Ravens, New York Giants, Oakland Raiders and Philadelphia Eagles without him to start the season.

The Broncos signed veteran outside linebacker Shaun Phillips to help account for the loss of Dumervil, but Phillips could now assume Miller's role instead. Denver coaches listed him as Miller's backup during offseason workouts, though Phillips did much of his work at defensive end.

A Broncos defensive player, who spoke to USA TODAY Sports on the condition of anonymity because a suspension has not been announced, said he was unaware of Miller having any issues with drugs.

"I don't think many people knew much," the player said. "No one talked about it."

It will certainly be a major issue when players report to training camp Wednesday. They will practice for the first time Thursday.

Miller is eligible to practice while his appeal is being heard. If suspended, he would be allowed to participate in camp and preseason games before starting a suspension on the first day of the first week of the regular season.

The news of Miller's suspension is just the latest in a string of bad news for the Broncos in recent weeks. Two of vice president John Elway's top personnel executives, Matt Russell and Tom Heckert, were arrested on separate incidents of drunk driving, and both have been suspended. Heckert, who joined the Broncos in May after being fired as general manager of the Cleveland Browns, was suspended for a month. Russell's suspension is indefinite.

The Broncos also had to deal with the embarrassing fashion in which they lost Dumervil, a three-time Pro Bowler and the 2009 NFL sacks leader. Dumervil had agreed to take a pay cut, but his revised contract was not received via fax before a hard deadline in which his 2013 salary would have become guaranteed. Rather than pay him $12 million, the Broncos cut him. Dumervil later chose to sign with the Baltimore Ravens.